"Undue influence" can happen when someone who benefits from a Will encourages the Testator to create the Will for the influencer's benefit. A common example is one child convincing his parents to leave him more in their Wills than his siblings. The siblings will be upset and may decide to challenge the Will.
In New Jersey, undue influence is exerted when a testator is coerced to do that which he would not have done if left to himself. The influence must be such that it destroys the Testator's free agency and causes him to dispose of his property not by his own desires, but instead by the desires of another.
While the challenger has the burden of proving that a Testator was subjected to undue influence, certain circumstances may create a presumption of undue influence and the burden of proof can be shifted to the party claiming that the Will is valid.
Challenging a Will can be a difficult proposition, but certainly not an insurmountable one when the facts and circumstances warrant it.
You should consult an experienced probate litigation attorney if you believe that a member of your family was subjected to undue influence.
Reference: My Prime Time News (Jan. 18, 2017) "Undue Influence."